1. Field of the Invention
The invention relates to a liquid crystal display, and more particularly to a reflection type liquid crystal display including an active matrix type display having a thin film transistor as a switching device, and a passive matrix type without a switching device. The invention also relates to a method of fabricating such a reflection type liquid crystal display.
2. Description of the Related Art
In recent years, a liquid crystal display has been widely used for a pocket-type TV set and terminal devices for communication by virtue of its small thickness and light weight. A reflection type liquid crystal display without necessity of using a back light is in particular in demand because it is ultra-thin and light-weight, and it can significantly reduce power consumption. However, even if a back light is removed out of a presently available transmission type color liquid crystal display and a light reflection plate is added to a lower surface of the display, it would cause a problem that an efficiency of utilizing lights is low, and it is not possible to have practical brightness.
As a solution to this problem, there have been suggested various reflection type liquid crystal displays for enhancing an efficiency of utilizing lights. For instance, a certain reflection type liquid crystal display is designed to include a pixel electrode having reflection function, and another is designed to include no polarizing plates.
For another instance, a reflection type liquid crystal display suggested in "Bright Reflective Multicolor LCDs addressed by a-Si TFTs" by S. Mitsui et al., Society for Information Display '92 Digest, pp. 437-440, is of a type where guest-host type liquid crystal including dichroism pigment is used as raw material for liquid crystal. FIG. 1 illustrates a structure of the suggested reflection type liquid crystal display.
The illustrated reflection type liquid crystal display includes an insulating substrate 41 and an opposing substrate 13 spaced away from the insulating substrate 41. A space between the substrates 41 and 13 is filled with liquid crystal 10. A color filter 12 is disposed on the substrate 13, and a transparent common electrode 11 is formed on the color filter 12. A gate electrode 9 is formed on the insulating substrate 41, and both the gate electrode 9 and the insulating substrate 41 are covered with a gate insulating film 8. A semiconductor layer 7 is formed on the gate insulating film 8 above the gate electrode 9. A source electrode 4 and a drain electrode 5 are also formed on the gate insulating film 8 in contact with the semiconductor layer 7. The source electrode 4, the drain electrode 5, the semiconductor layer 7, and the gate electrode 9 cooperate with one another to thereby constitute a bottom gate type thin film transistor (hereinafter, "thin film transistor" is referred to simply as "TFT").
An interlayer insulating film 42 is formed covering the source electrode 4, the drain electrode 5, the semiconductor layer 7, and the gate insulating film 8 therewith. A contact hole 44 is formed throughout the interlayer insulating film 42. A pixel electrode 43 made of aluminum is formed on both the interlayer insulating film 42 and an inner sidewall of the contact hole 44.
The drain electrode 5 of TFT is in contact with the pixel electrode 43 through the interlayer insulating film 42. As illustrated, the interlayer insulating layer 42 is designed to have a roughened surface by which the pixel electrode 43 acts as a reflection plate for diffusing lights therearound. Lights having been reflected from the pixel electrode 43 transmit through or are absorbed in the guest-host type liquid crystal layer 10.
The reflection type liquid crystal display illustrated in FIG. 1 remarkable enhances an efficiency of using lights by virtue that a polarizing plate is no longer necessary to be provided, and that the pixel electrode 43 acts as a light reflection plate.
What is important in the illustrated reflection type liquid crystal display is that the pixel electrode 43 as a light reflection plate is designed to have a roughened surface. If the pixel electrode 43 were designed to have a flat reflection surface, reflected lights would have more intense orientation, which causes an angle suitable for viewing a display (hereinafter, such an angle is referred to as "angle of visibility") to become narrower, and which in addition causes scenery to reflect at the pixel electrode 43 as it is like a plane mirror. As a result, there is caused a problem that a display cannot be seen well.
Thus, it is important that a light-reflection surface is roughened or designed to have projections and recesses to thereby uniformly scatter lights therearound. However, if the interlayer insulating layer 42 were designed to have projections and recesses to thereby provide the pixel electrode 43 with a plate for scattering and diffusing lights as suggested in the above-mentioned reflection type liquid crystal display illustrated in FIG. 1, it would be unavoidable for fabrication steps to become complicated, and as a result there would be caused a problem of an increase in fabrication costs.
In the above-mentioned reflection type liquid crystal display, the interlayer insulating film 42 is designed to have projections and recesses by photolithography. However, it is quite difficult to form and control a fine shape of the projections and recesses in micrometer order by photolithography, and hence the projections and recesses tend to become a trapezoid. In addition, there is another problem that it is not possible to have a uniform characteristic of reflection and scattering.
The reflection type liquid crystal display illustrated in FIG. 1 further has problems as follows. Firstly, it is quite difficult to form the contact hole 44 connecting the drain electrode 5 to the pixel electrode 43 through the interlayer insulating film 42. Secondly, at least six photolithography steps have to be carried out in order to form the active matrix substrate 41 on which TFT, the pixel electrodes 43 and so on are formed. In other words, six different masks are necessary to prepare, and exposure, development and etching steps have to be carried out six times, both of which causes an increase in a fabrication cost and a reduction in a fabrication yield. In addition, errors in registration between masks and the substrate have to be taken into account. As a result, it is quite difficult to form an active matrix substrate larger in size.
In order to solve the above-mentioned problems, Japanese Unexamined Patent Publication No. 4-338721 has suggested a reflection film made of a multi-layered dielectric film, for instance. The suggested reflection film brings an advantage that an efficiency of using lights can be enhanced by controlling reflectance of a mirror composed of the multi-layered dielectric film. However, since the mirror composed of the multi-layered dielectric film, acting as a light reflection plate, has a poor characteristic of light scattering, reflected lights have intense orientation, and accordingly a broader angle of visibility cannot be obtained.
Japanese Unexamined Patent Publication No. 7-306404 has suggested a liquid crystal display having a light diffusion film formed on a top surface of a transparent pixel electrode, and a plane reflection plate disposed at the rear of the transparent pixel electrode. However, in the suggested liquid crystal display, lights externally transmitted are scattered by the light reflection film formed on a top surface of the pixel electrode, before the lights are introduced into a liquid crystal layer. Hence, the suggested liquid crystal display cannot sufficiently provide display function utilizing birefringence of liquid crystal. In addition, a photolithography step has to be carried out seven times for fabricating the liquid crystal display, which causes problems of an increase in a fabrication cost and a reduction in a fabrication yield. It would be difficult to form a substrate larger in accordance with the suggested liquid crystal display.
As discussed above, the conventional liquid crystal displays have many problems. An advantageous process for readily fabricating a reflection surface having projections and recesses is to roughen an insulating substrate, namely, to use so-called ground glass. A reflection surface having projections and recesses could be readily fabricated, for instance, by forming an aluminum film on a roughened insulating substrate. However, if the reflection surface were fabricated in such a manner, a thin film transistor has to be fabricated on projections and recesses, as described in Japanese Unexamined Patent Publication No. 59-100488. Accordingly, halation tends to readily take place when a reflection plate is exposed to lights in a photolithography step, which would cause a problem of a design rule being restricted. In addition, there is another problem that the projections and recesses would cause deterioration in transistor performances and reduction in a fabrication yield.
As a solution to those problems, the Japanese Unexamined Patent Publication No. 59-100488 has also suggested that, in a substrate, an area on which TFT is to be formed remains flat, and only an area on which a pixel electrode is to be formed is formed to have projections and recesses. However, this is not an appropriate solution, because a quite complicated photo-etching step has to be carried out for remaining the first mentioned area flat. In addition, at least seven photolithography steps have to be carried out for fabricating an active matrix substrate, causing the problems as mentioned earlier. That is, a fabrication cost would be increased, and it would be difficult to form the active matrix substrate larger in size.
As mentioned above, the prior art has to carry out a complicated photo-etching step for fabricating projections and recesses at a surface of a substrate to thereby have a reflection plate having sufficient light diffusion performance, which would cause a problem of an increased fabrication cost. In addition, it would be quite difficult to control a reflection characteristic of the reflection plate.
Furthermore, a photolithography step has to be carried out at least six times for fabricating an active matrix substrate including a thin film transistor and a reflection plate. Hence, at least six different masks have to be prepared, and exposure and development steps have to be carried out at least six times. This causes an increase in a fabrication cost and a reduction in a fabrication yield. In addition, errors in registration between masks and a substrate have to be taken into account, resulting in that it is quite difficult to form an active matrix substrate larger in size.